1. Field of the Invention
The present invention is in the field of apparatus and methods to transmit an image over a narrow-band analog voice communication channel. More particularly, the present invention relates to apparatus and methods for providing a pair of users of the apparatus with full duplex voice communication; along with the capability to select a still image obtained from any one of a number of contemporary and commonly available image sources, and to transmit this still image from one user to the other user while voice communication is interrupted. Upon completion of the image transmission, receipt of the transmission, and display or storage of the image by the receiving user, full duplex voice communication is automatically restored. Many contemporary narrow-band analog voice communication facilities will support the present image transceiver apparatus and method, including common forms of the telephone (i.e., hard wired, cellular, mobile radio telephone, and others), short wave (high frequency SSB, mobile, and hand-held portable units), satellite links (INMARSAT-C, for example), and citizen's band (CB) radio.
2. Related Technology
For many years, short-wave amateur or "ham" radio operators have used a technology known as "slow-scan TV" to transmit still video images to one another over the narrow-band analog voice communication channel provided by their radio equipment. This technology uses sub-carrier FM (SCFM) modulation and demodulation, with the band-width of the modulated signal falling in the 1,200 Hz to 2,200 Hz audio band. In many cases, these instances of amateur short-wave radio operators transmitting video still images between one another over their radio equipment was accomplished by the use of custom-made equipment put together by the radio operators themselves. Consequently, the availability and commonality of this equipment was limited. Even though the slow-scan TV technology was in use, it was not available generally to the public, and the ability to transmit still video images over narrow-band analog telephone, or other voice communication channels, was not widely in use.
In recent years, some commercial equipment has become available to transmit images over narrow-band analog voice communication channels. Examples of this equipment is seen in U.S. Pat. Nos. 4,053,931, issued 11 Oct. 1977; 4,057,836, issued 8 Nov. 1977; 4,099,292, issued 4 Jul. 1987; 4,516,151, issued 7 May 1985; 4,564,823, issued 14 Jan. 1986; and 4,561,195, issued 17 Mar. 1987, which are all assigned to Robot Research, Inc. However, this equipment adopts the slow-scan TV technology virtually as it was used by the ham radio operators, and with the same limitations to its general use by the public. That is, conventional slow-scan TV equipment generally has the ability to accept as an image source only equipment of the broadcast type (b-camera, D1, etc), and not of the composite video type which is generally available to the public, such as common hetrodyne-type video cameras for consumer or industrial use (i.e., Beta, VHS, or 8 mm video tape recorder cameras, for example.
Further to the above, a consideration of this conventional equipment reveals that the video image signal is divided into its red/green/blue (RGB) image signal components before being modulated to form the slow-scan TV signal for transmission to another location. That is, the output of the video camera is decoded to extract the chroma signal, the luminescence signal, and the synchronization signal from the composite video signal without conversion to digital signal format. This is an analog signal separation process. The analog RGB signal is extracted, and is then converted to the digital signal format for storage in a video memory. This signal processing expedient (i.e., use of RGB signal division) imposes limitations on the conventional equipment which are not insignificant. Additional consideration of the conventional equipment shows apparently that considerable attention has in the past been given to the modulation and transmission of image signals using the slow-scan TV technology, but that little attention has been given to the demodulation of the image signal and its conversion back to a form which can be stored or displayed on commonly available equipment. Consequently, the signal demodulation technique used to a zero-crossing detection with use of a limiter circuit (i.e., quadrature detection). Also, the signal that is acquired by timing synchronized to the sub-carrier of the hetrodyne type VTR (i.e., the consumer type 8 mm, Beta, and VHS equipment) which has no compensation by a time base corrector to correct the time axis of the signal, can cause jitter of the received still image. These and other shortcomings of the conventional slow-scan TV technology are especially evident when considered in light of the marginal operating conditions which frequently apply with narrow-band voice communication channels.
That is, while speaking to one another under marginal operating conditions of a voice communication channel, users of the channel may be bothered only a little or not at all by the poor quality of the voice communication. Although there may be some loss of voice quality, tone, or fidelity, the human hearing is adaptable and can compensate somewhat for the deficiencies of the communication channel. Moreover, these users will still be able to converse with one another quite successfully. On the other hand, the quality of image transmission which can be achieved with conventional slow-scan TV equipment under such conditions will leave many users dissatisfied. Nevertheless, the image throughput rate of slow-scan television technology is generally better than that of digital image signal transmission technology over the marginal channel condition afforded by common narrow-band analog voice communication channels. That is, digital image transmission technology would require a slow transmission rate or would provide unsatisfactory results with a marginal analog voice transmission channel connecting the users of such a system. That is, conventional slow-scan TV techniques give a better throughput rate under marginal operating conditions of a narrow band analog voice communication channel than do digital image transmission techniques. Accordingly, this limited slow-scan TV technology continues in use, but is not generally used by the public at large.